Apologies for not being able to respond sooner.
Mathematica will prompt for an API key upon first time use of certain services like OpenAI.
Yes, Wolfram Language will prompt you for an API key to access the OpenAI API that allows using the LLM in a Notebook.
one can verify which services have been registered in Mathematica e.g. through SystemCredentialKeys one can view which keys have been registered. Or specifically what key has been registered e.g. SystemCredential["OPENAIAPIKEY"]
Chat Notebooks and the LLMFunctions paclet (this includes functions like LLMSynthesize, ImageSynthesize, ChatEvaluate, etc.) use the API slightly differently internally. Chat Notebooks really only looks for SystemCredential["OPENAIAPIKEY"] where the LLMFunctions paclet can look at SystemCredential, an OpenAI service connection, an environment variable, and you can explicitly set your API key in the programmatic call.
How to manually set or change any API key required for a service like OpenAI i.e. SerrviceConnect["OpenAI","New"]. Some users may have clicked away the Key registration form at the initial start of a service.
If you're only using Chat Notebooks, the SystemCredential method is the way to go. If you have used or will use LLMFunctions paclet (this is new 13.3 symbols like LLMSynthesize, see: https://reference.wolfram.com/language/guide/LLMFunctions.html) then you will want to use ServiceConnect["OpenAI", "New"] to create a new connection. You can see a list of saved connections by doing
ServiceConnections`SavedConnections[]
and you can delete a specific connection by doing
ServiceConnections`DeleteConnection[]
You only need to be able to create a valid API key from OpenAI for the services to work within Wolfram Language. OpenAI has documented what ChatGPT Plus gives you here: https://help.openai.com/en/articles/6950777-what-is-chatgpt-plus
Is the ChatGPT API included in the ChatGPT Plus subscription?
No, the ChatGPT API and ChatGPT Plus subscription are billed separately. The API has its own pricing, which can be found at https://openai.com/pricing. The ChatGPT Plus subscription covers usage on chat.openai.com only and costs $20/month.